The need for miniaturized power sources for micro-electro-mechanical systems (MEMS) and micro-electronics has long been recognized. Much work has already been done on micro-scale batteries, and micro-scale heat engines. Micro-scale heat engines are a particularly attractive option, because of the very high density energy storage afforded by the hydrocarbon fuels they burn. Thus, a micro-heat engine which could convert the chemical energy stored in a hydrocarbon fuel to mechanical or electrical energy could form the basis of a very compact power supply.
Attempts to construct a micro-heat engine have been directed to redesigning the hardware for conventional large-scale heat engines to be fabricated on the micro-scale. So far, these attempts are still in the developmental stages and have not yet proven to be successful.
Although not true heat engines, thermally driven devices have been constructed on the micro-scale. In one such device, water contained in a cavity, when heated, pushes a drive shaft. Upon cooling, the water condenses and the capillary action of the liquid pulls the drive shaft back in. However, because the device uses electrical power to heat and vaporize the working fluid and because there is no provision to cyclically heat and cool the working fluid, the device functions as an actuator, not as a true heat engine. Other thermally driven actuators rely on the thermal expansion of solids, or the change in shape of so-called shape-memory alloys to exert forces over small distances. Again, these devices are not true heat engines because they do not operate cyclically, and they do not transform thermal energy into mechanical or electrical power.
Piezoelectric thin films have been used for years as power transducers in MEMS and micro-electronic devices. Piezoelectric films are an attractive option for power transduction because of the relative ease with which such devices can be produced using conventional micro-machining methods. Generally speaking, micro-machining involves processing techniques, such as micro-lithography and etching, that were developed and refined for use in the manufacture of integrated circuits. Micro-machining allows fine control of dimensions and is commonly employed for producing parts from silicon. However, micro-machining is not restricted in its application to the formation of workpieces from silicon or other materials conventionally used in the manufacture of integrated circuits, and it is known to apply micro-machining to other materials.
In most applications of piezoelectric films, such as in micro-actuators, pumps and valves, electrical power is converted to mechanical power. Micro-sensors that utilize piezoelectric films also have been used for mechanical to electrical transduction, however, such devices are not capable of producing usable electrical power to any significant degree. Thus, it would be desirable to utilize piezoelectric thin films for converting energy in one form, such as thermal energy or kinetic energy, to useful electrical energy to power MEMS and micro-electronic devices.
Along with the need for miniaturized power sources is the need for micro-devices that are designed to remove heat from MEMS and micro-electronics. In particular, integrated circuit manufacturers are already reaching limits on micro-processor speed and performance imposed by high operating temperatures. Consequently, reducing chip operating temperatures by removing waste heat through active cooling is considered to be among the most promising strategies available to the micro-processor industry for overcoming these obstacles. Thus, it would be desirable to implement a piezoelectric film in a micro-heat pump for cooling applications of MEMS and micro-electronics.
Therefore, there exists a strong need for piezoelectric micro-transducers for use with MEMS and micro-electronic devices.